An image sensor for an electronic imaging device such as a camera generally consists of an array of photosensitive picture element detectors (“pixel detectors”). Light falling on the image sensor is detected by the pixel detectors, which generate output signals corresponding to the amount of light falling on each of those detectors. The output signals of the pixel detectors are digitised and stored in an electronic file that contains the image information. The image sensor may be incorporated into either a still camera for taking single images, or a video camera, or any other electronic imaging device.
Many electronic imaging devices use a CCD sensor, which measures the incident light by integrating the photocurrent of each of the pixel detectors over a predetermined period to obtain a measurement of the charge that has passed through the detector. The huge market for low cost electronic imaging sensors that can be incorporated in devices such as digital cameras, mobile telephones and personal computers has also spurred the development of single chip CMOS sensors, which have several advantages over the existing CCD sensors. However, the vast majority of CMOS sensors use the same sensing strategy as CCD sensors: they integrate the photocurrent.
Integrating the photocurrent within each pixel works well under relatively uniform illumination conditions, where the luminance of the image subject has a relatively low dynamic range. However, natural scenes can have a very large dynamic range: for example of six decades. This causes a problem for image sensors with conventional integrating pixels, which have a linear output of relatively low dynamic range.
It is possible to capture high dynamic range scenes with low dynamic range cameras by using multiple integration times and then creating a composite image. However, this is necessarily a slow process, which is unsuitable for many applications (for example, video capture).
An alternative method of extending the dynamic range of linear pixel detectors has been devised by Stoppa et al (David Stoppa, Andrea Simoni, Lorenzo Gonzo, Massimo Gottardi and Gian-Franco Dalla Betta: ‘Novel CMOS Image Sensor with a 132-dB Dynamic Range’ IEEE JSSC 37(12) 1846-1852 (2002)). To achieve an increase in dynamic range, a comparator is integrated into each pixel. The comparator compares the voltage within the pixel with a threshold voltage. If the pixel voltage reaches the threshold value, the comparator disconnects two capacitors in the pixel from two analogue input voltages that together represent the time at which this event occurs. At the end of the integration process, the pixel voltage and the two time voltages are sampled from each pixel. These three analogue voltages are then digitised to 8-bits each, creating a 24-bit signal that encodes the photocurrent within the pixel.
The large number of bits per pixel produced by the above system is typical of the results of trying to represent a high dynamic range signal in a linear format. In the case of a camera the problem is compounded by the fact that the aim should be to match the performance of the human visual system, which means that the camera should be sensitive to 1% changes in luminance.
It is recognised that using a logarithmic scale is often a good strategy for representing a high dynamic range signal. In the case of a camera there may also be other fundamental reasons why this is helpful:    (i) The physical process of scene formation means that the dynamic range of a scene is dominated by illumination variations whilst the information is contained in images of the reflectance of objects. Since these two quantities are multiplicative, generating a logarithmic representation is the first critical step in the majority of processes, such as homomorphic filtering and tone mapping, whose object is to reduce the impact of illumination variations so that objects can be recognized or scenes displayed on devices with low dynamic ranges.    (ii) The human visual system is sensitive to contrast changes of approximately 1% over a wide range of illuminations. A logarithmic format for the image captures this information in the fewest possible bits. For example using a logarithmic format a 1% change in four decades can be represented by a signal with a dynamic range of a thousand rather than a million in linear format. This dramatic reduction in dynamic range significantly simplifies the design of the camera's electronics and reduces the amount of data generated.
These advantages suggest that a useful approach to achieving imaging of high dynamic range scenes is to use an image sensor with a logarithmic response. This may be particularly useful in applications involving video-rate capture on single chip cameras.
It appears therefore that high dynamic range logarithmic image sensors should be ideally suited to use in camera systems that are required to image natural scenes. However, despite the potentially huge market for a high dynamic range camera, logarithmic cameras remain relatively obscure and undeveloped. The reason for this is that, like ‘silicon retinas’, existing logarithmic cameras use a MOSFET operating in subthreshold within each pixel detector to create an output voltage that is proportional to the logarithm of the photocurrent in each pixel detector. Although this circuit has the correct functionality, the mechanism has two major problems. The first problem is that there are large variations between the characteristics of individual MOSFETs operating in subthreshold. The resulting variability in pixel response, known as fixed pattern noise, can be equivalent to changing the photocurrent by an order of magnitude. Although techniques have been devised to improve the quality of output images by correcting for fixed pattern noise, the other major problem remains. This is that the output voltage typically changes by less than 60 mV when the photocurrent changes by an order of magnitude. The maximum output signal change is thus only 0.3V. This means that the output signals from the pixel detectors are susceptible to temporal noise.
Two techniques have been proposed to increase the dynamic range of the output voltage from a logarithmic pixel. The first of these is based upon use of a floating-gate device in the place of the load transistor (S. Collins, J. Ngole and G. F. Marshall “A High Gain Trimmable Logarithmic CMOS Pixel” Electronics Letters, 36, (21) 1806 (2000)). Although this approach increases the dynamic range of the output voltage it relies upon immature floating-gate device technology. In addition, the increase in voltage swing within the pixel emphasises the slow response time already observed in logarithmic pixels when the photocurrent suddenly decreases.
An alternative approach that claims to increase the output voltage swing of a logarithmic pixel has been proposed by Lai, Lai and King (“A Novel Logarithmic Response CMOS Image Sensor With High Output Voltage Swing and In-pixel Fixed-Pattern Noise Reduction” Liang-Wei Lai, Cheng-Hsiao Lai and Ya-Chin King IEEE Sensors Journal 4(1) 122-126 (2004)). This sensor uses a bipolar phototransistor rather than a photodiode in an otherwise conventional logarithmic pixel. Although this change increases the output voltage swing of the pixels, this increase occurs in a region in which the response is not logarithmic. A closer examination of the characteristics of the pixel detector shows that the response is not a simple logarithmic function of the illumination intensity. In fact, the bipolar transistor amplifies the photo current, and at high illumination intensities the load transistor is driven into moderate inversion, rather than the weak inversion that is required to obtain a logarithmic response.
In summary, there appears to be a distinct advantage to using a pixel with a logarithmic response to create an image sensor suitable for capturing high dynamic range scenes. However, all existing pixels with this type of response either suffer from fixed pattern noise, or rely on unproven technology, or provide only an approximately logarithmic response.
It is an object of the present invention to provide an image sensor that mitigates at least some of the aforesaid problems.